1. Field of the Invention
The present invention relates to a probe for detecting a transient magnetic resonance signal induced in a sample, wherein the ratio of the Q of the probe to the Q of the resonance signal is relatively large, and preferably larger than 1.
2. Description of the Related Art
Magnetic resonance is useful to detect the presence of a specific substance in a sample. For example, generally, radio frequency (RF) radiation at a particular frequency will induce a magnetic resonance signal in a specific substance, but not in other substances. Therefore, the induced magnetic resonance signal can be detected to thereby indicate the presence of the specific substance.
It is common to detect a magnetic resonance signal by placing a sample to be measured in a tuned, electronically resonant tank circuit. Then, the response of the tank circuit to the electromotive force produced by nuclear or electronic spins in the sample is measured. With Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR), the sample is placed in or near an inductor. Parallel and/or series capacitance is added to make the circuit electrically resonant at the measurement frequency. One or more additional reactive impedances (inductors or capacitors) are typically added to adjust the resistive impedance at resonance to a particular value which optimizes the detection sensitivity.
FIG. 1 is a diagram illustrating an example of a conventional magnetic resonance apparatus. Referring now to FIG. 1, a transmitter 20 and a receiver 22 are connected to a probe 24 through a transmit/receive (T/R) switch 26. Probe 24 includes an inductor, such as a coil 28, forming part a resonant, tuned tank circuit with various other inductors L and capacitors C. To detect the presence of a target substance, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Then, transmitter 20 generates a pulse and supplies the pulse to probe 24. As an example, in NQR, the pulse is formed by an RF signal having a frequency corresponding to the resonance signal of the target substance which is intended to be detected. Probe 24 receives the pulse, which causes coil 28 to store (RF) energy.
If a sample (not illustrated) is appropriately placed near, or inside, coil 28, the stored RF energy will cause a corresponding RF magnetic field to irradiate the sample. If the sample includes the target substance, the RF magnetic field will induce a magnetic resonance signal in the target substance. For example, if the apparatus operates under the principles of NMR, then an appropriate NMR resonance signal will be induced. If the apparatus operates under the principles of NQR, then an appropriate NQR resonance signal will be induced.
After the sample is irradiated with the RF magnetic field, T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. Coil 28 then detects the resonance induced in the target substance, and probe 24 produces a corresponding output signal. The output signal of probe 24 is received and analyzed by receiver 22, to confirm the presence of the target substance in the sample.
FIG. 1 is only one example of a magnetic resonance apparatus. For example, FIG. 1 illustrates T/R switch 26 to connect transmitter 20 and receiver 22 to the same probe 24. However, instead, a transmitter and receiver can each have a separate, dedicated probe together with a switch or gate for protecting the receiver while the transmitter is ON.
A probe and a resonance signal induced in a sample each have a corresponding, respective quality factor, Q. The Q of the probe is actually the Q of the tuned tank circuit of the probe. Therefore, the xe2x80x9cQ of the probexe2x80x9d and the xe2x80x9cQ of the tuned tank circuit of the probexe2x80x9d are used interchangeably herein. Further, the bandwidth of the probe is defined as f/Q, where f is the resonant frequency of the probe.
Magnetic resonance measurements are largely concerned with the measurement of the spectrum of the nuclear (or electronic) spins in the sample. Therefore, conventionally, the bandwidth of the probe is greater than the bandwidth of the detected magnetic resonance signal so that the entire magnetic resonance signal falls within the bandwidth of the probe. These relative bandwidths of the probe and the magnetic resonance signal allow the tuned tank circuit of the probe to faithfully follow (that is, reproduce) the magnetic resonance signal.
To achieve these relative bandwidths, the Q of the probe must be small enough so that the bandwidth (f/Q) of the probe is larger than the bandwidth of the magnetic resonance signal. In this case, as described above, the probe can follow the magnetic resonance signal.
Moreover, it is well-known that when the Q of the probe is small enough to follow the magnetic resonance signal, the SNR of the apparatus increases in proportion to the square root of Q. Therefore, generally, it is desirable to increase the Q of the probe as high as possible, while still allowing the probe to follow the magnetic resonance signal.
To summarize, the Q of probe should be as high as possible to increase the SNR of the apparatus, but low enough to provide a probe bandwidth (f/Q) which is greater than the bandwidth of the magnetic resonance signal. In most magnetic resonance applications, such as in NQR, Qxe2x80x2 is conventionally less than 0.3, and is typically much less than 0.3, where Qxe2x80x2 is defined as the ratio of the Q of the probe to the Q of the magnetic resonance signal. Here, the Q of the signal is defined as the ratio of the signal""s center frequency to its bandwidth. Increasing Qxe2x80x2 above 0.3 reduces the bandwidth of the probe with respect to the bandwidth of the magnetic resonance signal so that the probe cannot accurately follow the magnetic resonance signal. If the probe cannot accurately follow the magnetic resonance signal, it is conventionally believed that the SNR will be reduced. Therefore, in most magnetic resonance applications, such as in NQR, Qxe2x80x2 of 0.3 conventionally places a limit on the Q of the probe with respect to the Q of the magnetic resonance signal.
There are some cases where the condition of Qxe2x80x2=1 is approached. These occur in magnetic resonance imaging with very large field gradients, and in some instances of wideline magnetic resonance spectroscopy. In these cases, the intrinsic magnetic resonance signal linewidth is greater than the probe bandwidth. However, since the probe is also used to excite the magnetic resonance signal, the Q of the signal is greater than or equal to the Q of the detector, leading to Qxe2x80x2 less than =1.
Therefore, the SNR of a conventional apparatus for detecting magnetic resonance signals will be limited by the allowed value of Qxe2x80x2. This limitation on the SNR may not be acceptable for detecting magnetic resonance signals in certain situations.
Accordingly, it is an object of the present invention to provide an apparatus which detects a magnetic resonance signal, and has an improved detection ability and SNR over conventional apparatuses.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing an NQR apparatus which includes a probe for detecting an NQR signal induced in a sample. The probe and the NQR signal each have a respective, corresponding Q, and the ratio, Qxe2x80x2, of the Q of the probe to the Q of the NQR signal is greater than, or equal to, 0.3.
Objects of the present invention are also achieved by providing an apparatus which includes a probe for detecting a resonance signal induced in a sample. The probe and the resonance signal each have a respective, corresponding Q, and the ratio, Qxe2x80x2, of the Q of the probe to the Q of the resonance signal is greater than 1.